TW202412047A - Anti-scanning operation mode of secondary-electron projection imaging system for apparatus with plurality of beamlets - Google Patents

Abstract

A method of operating a secondary imaging system of a charged particle beam apparatus may include using an anti-scanning mode.Excitation of a component of the secondary imaging system may be adjusted synchronously with a primary scanning deflection unit.Together with an anti-scanning deflection unit performing anti-scanning, a component of the secondary imaging system, such as a lens, may be adjusted in step.As scanning and anti-scanning is performed, excitation parameters of the component may also be constantly updated.

Description

Backscanning mode of operation of a secondary electron projection imaging system for a device having a plurality of beamlets

The description herein is related to a charged particle device that may have a plurality of charged particle beams. More particularly, the description herein is related to a device that uses a plurality of charged particle beams (beamlets) to simultaneously acquire images of a plurality of scanned regions of an observation area on a sample surface. This device can be used to detect or inspect defects on wafers or masks with high resolution and high throughput, which is applicable to the semiconductor manufacturing industry.

In the manufacturing process of integrated circuits (ICs), unfinished or finished circuit components can be inspected to ensure that they are manufactured according to design and are free of defects. Inspection can be performed by a charged particle beam system that scans (e.g., deflects) a primary beam across a sample and collects secondary particles generated from the sample at a detector. An example of a charged particle beam system is a scanning electron microscope (SEM). SEMs use electron beams because such beams can be used to see structures that are too small to be seen by optical microscopes (such as microscopes that use visible light).

Some SEM systems may use multiple beams (e.g., beamlets) to improve throughput. Multiple primary beams may be scanned across sub-areas on a sample, and multiple beams of secondary particles may be generated from the sample and directed to a secondary imaging system. The secondary imaging system may perform a backscan to account for adjustments to the position of the secondary beam due to the scan of the primary beam. However, components in the secondary imaging system may not properly account for the scanning and backscanning of the beam. For example, components in the secondary imaging system including lenses and aberration compensation elements may be optimized for use only in an undeflected position of the beam. When a beam is deflected due to scanning or backscanning, the beam spot on the detector may be defocused and imaging quality may be degraded.

Embodiments of the present invention provide systems and methods for charged particle beam-based imaging. In some embodiments, methods for adjusting focus or controlling aberrations in a secondary imaging system may be provided. The method may include correcting the focus of a secondary beamlet that is defocused due to backscanning deflection. The method may include adjusting the excitation of components of the secondary imaging system based on the deflection of a backscanning deflector.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosed embodiments as may be claimed.

Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the drawings. The following description refers to the accompanying drawings, wherein the same reference numerals in different drawings represent the same or similar elements unless otherwise indicated. The implementations described in the following exemplary embodiment descriptions do not represent all implementations consistent with the present invention. Instead, they are merely examples of devices, systems, and methods that are consistent with aspects of the subject matter that may be described in the attached patent claims.

Electronic devices consist of circuits formed on a block of silicon called a substrate. Many circuits can be formed together on the same block of silicon and are called an integrated circuit or IC. As technology has advanced, the size of these circuits has been reduced dramatically, allowing many of them to fit on a substrate. For example, in a smartphone, an IC chip can be the size of a thumbnail and can include over 2 billion transistors, each less than 1/1000 the size of a human hair.

Manufacturing these tiny ICs is a complex, time-consuming, and expensive process that often involves hundreds of individual steps. An error in even one step can cause a defect in the finished IC, rendering it useless. Therefore, one goal of the manufacturing process is to avoid such defects in order to maximize the number of functional ICs manufactured in the process, i.e., to improve the overall yield of the process.

One component of improving yield is monitoring the chip manufacturing process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at different stages of their formation. Inspection can be done using a scanning electron microscope (SEM). The SEM can be used to actually image these extremely small structures, thereby obtaining an "image" of the structure. The image can be used to determine whether the structure was formed properly, and also whether the structure was formed in the proper location. If the structure is defective, the process can be adjusted so that the defect is less likely to recur.

An image of a wafer can be formed by scanning a primary beam of an SEM system over the wafer and collecting particles (e.g., secondary electrons) generated from the wafer surface at a detector. The imaging process can include focusing the primary beam to a point and deflecting (e.g., bending) the beam so that it passes over an area of the wafer in a line-by-line pattern (e.g., grating scanning). At a given time, the beam can be focused to a specific location on the wafer, and the output of the detector at that time can be associated with that specific location on the wafer. An image can be reconstructed based on the detector output at each pass along the beam scan path.

Inspecting wafers using a single primary beam of an SEM system can be time consuming. To improve throughput, a multi-beam SEM system can be used. Multiple primary beams (e.g., multiple beamlets) (rather than just a single beam that scans across the wafer) can be formed in an array (e.g., a grid pattern) and can be scanned together across the wafer. Multiple beams of secondary particles can be generated at the wafer, and the secondary particles can be directed to a detector via a secondary imaging system. The detector in a multi-beam system can include an array of detector cells (e.g., different sub-regions of the detector, or different groups of sensing elements), each configured to receive secondary particles associated with one of the multiple beams.

In a multi-beam system, as multiple primary beams are scanned across a wafer, the positions of the multiple beams of secondary particles may be shifted, such as by a deflector that deflects the beams to achieve the scan across the wafer. Backscan deflectors may be provided in the secondary imaging system to compensate for this shift and minimize image displacement on the detector. That is, as the primary beams are deflected by scanning, the beams of secondary particles are reversely deflected by the deflectors of the secondary imaging system through a process known as "backscanning." These backscan deflectors can help keep the secondary particles of each beam received in its respective detector unit (e.g., maintaining a one-to-one correspondence between beams and detector units).

A secondary imaging system may be provided between the wafer and the detector and may include components that help improve image quality. For example, there may be a zoom lens for adjusting magnification so that the beam spot is formed at the appropriate size on the detector unit, a derotating lens for adjusting the rotational position of the beam, and an aberration compensation element for compensating for various aberrations. However, these components are typically fixed to a specific operating state during scanning. These components may be set to provide the best average performance for multiple beams throughout the scanning process and not adjusted during scanning.

However, while a scan is being performed, and even while a backscan is also being performed, the properties of the multiple beams may change. For example, the origin position of the beam of secondary particles changes constantly during the scan, and the path of the beam toward the detector also changes. Therefore, although the components in the secondary imaging system may be set for a basic state (e.g., optimized for an average state or an undeflected state), some degradation in image quality may occur during the scan because the beam may be temporarily in a state for which the secondary imaging system is not set or optimized. Such degradation may be more pronounced at the ends of the scan range. For example, in a line-by-line scan pattern, the deflection of the beam may be greatest at the corners of the pattern. At the same time, the components of the secondary imaging system can be optimized for focusing the beam when the beam is in a central (undeflected) position, or can be optimized for averaging of deflected states. When the secondary imaging system is configured in this manner, distorted beam spots can form on the detector when the beam is deflected (e.g., at each scanning phase). Such distortions can lead to low collection efficiency (e.g., the ratio of collected secondary particles to generated secondary particles), and undesirable crosstalk (e.g., where particles from different beams are mixed).

To enhance the performance of SEM systems, it would be desirable to compensate for the different properties of multiple beams in a secondary imaging system as they scan across a sample surface.

Embodiments of the present invention can solve problems such as those discussed above by operating the components of the secondary imaging system so that they are adjusted in synchronization with the scanning of the primary beam. The components of the secondary imaging system can be adjusted synchronously with the scanning deflector or the backscanning deflector. For example, along with the backscanning, the secondary imaging system can be adjusted to the excitation of other components (such as a zoom lens, a projection lens, or an aberration compensation element). The adjustment of the components of the secondary imaging system can be performed together with the backscanning performed by the backscanning deflector, and therefore the adjustment of the components of the secondary imaging system can be referred to as the "backscanning mode" of the secondary imaging system.

The beam may be deflected due to the scan, and its properties may change. Together with this, the secondary imaging system may adjust the excitation applied to various components in the secondary imaging system based on the deflection so that the imaging conditions at any given scan position are improved or even optimized. The beam spot may be formed on the detector with improved imaging quality. For example, the beam spot may be formed in a manner that avoids or reduces undesirable distortions, or may be formed so that a desired shape may be achieved. The beam spot may be formed so that secondary particles from the beam are contained in the detector cell. Collection efficiency may be improved when a larger proportion of secondary particles reach the intended detector cell. In addition, crosstalk may be reduced when secondary particles of one beam are prevented from reaching detector cells associated with different beams.

In some embodiments, optimization of the beam spot on the detector surface can be targeted to a specific shape, which can be different from a circle, such as an elliptical shape. The detector unit can be square, and the typical target beam spot can be circular to ensure that the beam spot fits the detector unit. However, when multiple detector units are provided, such as in a multi-beam system, deformation of some of the beam spots can deform the circular beam spot and can lead to negative effects such as poor collection efficiency and crosstalk, as discussed above. On the other hand, the elliptical shape of the beam spot can help maximize collection efficiency and minimize crosstalk. The orientation of the elliptical shape can be diagonal. For example, the elliptical shape can be oriented on a diagonal of a square detector unit. The orientation may be based on the configuration of adjacent detector units. The secondary imaging system may be configured so that the beam spot has an elliptical shape. A "backscan mode" of the secondary imaging system may target an elliptical shape of the beam spot on the detector surface. Embodiments may enhance collection efficiency and reduce or eliminate detector crosstalk.

The objects and advantages of the present invention can be achieved by the elements and combinations illustrated in the embodiments discussed herein. However, it is not necessary for the embodiments of the present invention to achieve such exemplary objects or advantages, and some embodiments may not achieve any of the stated objects or advantages.

Without limiting the scope of the invention, some embodiments may be described in the context of providing systems and methods in a system utilizing an electron beam ("e-beam"). However, the invention is not limited thereto. Other types of charged particle beams may be similarly applied. In addition, systems and methods associated with backscanning may be used in other imaging systems, such as optical imaging, photon detection, x-ray detection, ion detection, etc. In addition, distortion control or beam projection may be applicable in other systems, such as lithography systems. In addition, the term "beamlet" may refer to a portion of a beam or a separate beam extracted from an original beam. The term "beam" may refer to a beam or a beamlet.

The term "detector cell" may refer to a portion of a detector. Individual detector cells may be configured to detect charged particles associated with a beamlet. The detector may be pixelated, having a plurality of sensing elements in an array pattern. One or more of the sensing elements may be grouped together and form a detector cell. A sensing element may refer to a semiconductor diode that makes up the detector. A sub-region of detection may correspond to a detector cell.

As used herein, unless otherwise specifically stated, the term "or" encompasses all possible combinations unless otherwise feasible. For example, if a component is stated to include A or B, then unless otherwise specifically stated or not feasible, the component may include A, or B, or A and B. As a second example, if a component is stated to include A, B, or C, then unless otherwise specifically stated or not feasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

Referring now to FIG. 1 , an exemplary electron beam inspection (EBI) system 10 that may be used for detection consistent with embodiments of the present invention is illustrated. The EBI system 10 may include a scanning electron microscope (SEM) and may be used for imaging. As shown in FIG. 1 , the EBI system 10 includes a main chamber 11, a load/lock chamber 20, an electron beam tool 100, and an equipment front end module (EFEM) 30. The electron beam tool 100 is located within the main chamber 11. The EFEM 30 includes a first load port 30a and a second load port 30b. The EFEM 30 may include additional load ports. The first loading port 30a and the second loading port 30b receive front opening unit pods (FOUPs) containing wafers (eg, semiconductor wafers or wafers made of other materials) or samples to be inspected (wafers and samples may be collectively referred to as "wafers" herein).

One or more robots (not shown) in the EFEM 30 transfer the wafer to the load/lock chamber 20. The load/lock chamber 20 is connected to a load/lock vacuum pump system (not shown), which removes gas molecules in the load/lock chamber 20 to achieve a first pressure lower than atmospheric pressure. After reaching the first pressure, one or more robots (not shown) can transfer the wafer from the load/lock chamber 20 to the main chamber 11. The main chamber 11 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in the main chamber 11 to achieve a second pressure lower than the first pressure. After reaching the second pressure, the wafer undergoes inspection by the electron beam tool 100. The electron beam tool 100 can be a single beam system or a multi-beam system. The controller 109 is electronically connected to the electron beam tool 100 and can also be electronically connected to other components. The controller 109 can be a computer configured to implement various controls for the EBI system 10. Although the controller 109 is shown in FIG. 1 as being outside the structure including the main chamber 11, the load/lock chamber 20, and the EFEM 30, it should be understood that the controller 109 can be part of the structure.

Charged particle beam microscopes, such as those formed by or that may be included in the EBI system 10, may be capable of resolution, for example, to the nanometer scale, and may serve as a practical tool for inspecting IC components on a wafer. Using an electron beam system, electrons of a primary electron beam may be focused at a probe spot on an inspected wafer. The interaction of the primary electrons with the wafer may cause the formation of a secondary particle beam. The secondary particle beam may include backscattered electrons, secondary electrons, or Ogier electrons, etc., generated by the interaction of the primary electrons with the wafer. The characteristics of the secondary particle beam (e.g., intensity) may vary based on the properties of the internal or external structure of the wafer, and may therefore indicate whether the wafer includes defects.

The intensity of the secondary particle beam can be determined using a detector. The secondary particle beam can form a beam spot on the surface of the detector. The detector can generate an electrical signal (such as current, charge, voltage, etc.) representing the intensity of the detected secondary particle beam. The electrical signal can be measured using a measurement circuit system, which may include other components (such as analog-to-digital converters) to obtain the distribution of the detected electrons. The electron distribution data collected during the detection time window combined with the corresponding scan path data of the primary electron beam incident on the wafer surface can be used to reconstruct an image of the inspected wafer structure. The reconstructed image can be used to reveal various features of the internal or external structure of the wafer, and can be used to reveal defects that may exist in the wafer.

Some embodiments of the present invention may be related to the design and operation of a multi-beam charged particle device that can utilize multiple charged particle beams. One of the implementations of such a device is a high resolution imaging system using the principles of a multi-beam SEM. In particular, a multi-beam SEM can be implemented as a wafer inspection tool and applied to imaging circuit patterns manufactured on semiconductor wafers. Single-beam SEMs are known to be widely used in the semiconductor industry as wafer inspection tools. The primary advantages of SEMs can be their high resolution and high sensitivity to surface charge distribution, which makes the SEM sensitive to a variety of manufacturing defects. However, for surface imaging, these tools utilize a single electron beam that scans across the inspection area, and therefore they are greatly affected by low image acquisition speeds. In a multi-beam SEM, to increase the measurement speed, the inspection area can be split into multiple sub-areas that are all scanned simultaneously by multiple focused electron beams. Such systems can be applied in the semiconductor industry as high-throughput and high-resolution inspection tools for wafer and mask inspection.

In the semiconductor industry, pattern analysis, defect detection, critical dimension measurement, and process control on semiconductor wafers can be important in the development of any manufacturing process for microchips. Even for mature processes, defects associated with different lithography steps on semiconductor wafers can pose challenges to high-volume manufacturing. Therefore, process control based on the number and type of defects and continuous detection of the number and type of defects can be important.

Wafer inspection systems help manufacturers detect defects that occur during the manufacturing process, thereby allowing them to monitor and control the process and increase the yield of integrated circuit (IC) chips.

Optical inspection systems are common industry tools for performing wafer defect inspection, critical dimension measurement and whole silicon wafer metrology in comparison systems. However, the development of the semiconductor industry in recent decades and the tremendous growth in the packing density of individual components in microchips has led to a significant reduction in component size. This is evident from the reduction in node size between microchip generations during recent decades. For the latest generations of microchips, the node size has reached a scale of only a few nanometers.

To detect low nanometer-sized nodes, optical inspection systems have fundamental limitations in imaging resolution and defect sensitivity inherited from the optics. The resolution of the system can be limited by the relatively large wavelength of light compared to the node size. Sensitivity can be limited by the low scattering cross-section of objects with sub-wavelength dimensions, which limits the intensity of the backscattered light.

These limitations reduce the sensitivity and throughput of optical inspection tools, making them less and less used for wafer analysis. This has led to the widespread adoption of inspection tools based on the principles of electron optics. These tools, which use the principles of scanning electron microscopes, provide high imaging resolution and sensitivity to the local charge distribution of semiconductor wafers, which can be important for defect detection.

Scanning electron microscopes can be used to image sample surfaces using a beam of accelerated electrons focused down to a few nanometers. High energy electrons incident on the surface interact with surface atoms, producing probe light spots that emit secondary and backscattered electrons. Images of the surface and local surface components can be obtained by collecting and analyzing these secondary and backscattered electrons. In detail, the recorded signal intensity of the secondary (or backscattered) electrons versus the position of the primary electron beam can be used to obtain an image of the detected surface. This allows the SEM to be applied to observe fine details of semiconductor circuit structures fabricated on wafers. The sensitivity of the acquired image to the charge distribution on the wafer surface allows the detection of local charging of the surface. SEM can be sensitive to defects in the fabricated semiconductor structures.

It is known that a single beam SEM may include two main sections: (i) a primary electron column; and (ii) a secondary electron column (or, in short, a detector for secondary electrons). The primary and secondary columns are named corresponding to the primary and secondary electrons. Primary electrons may include electrons originating from an electron source of the SEM, transported to a sample surface (e.g., a wafer), and incident on the sample surface. Secondary electrons may include: (i) low energy secondary electrons emitted from the wafer surface (having a Lambert's law electron energy distribution); (ii) backscattered electrons (elastically or inelastically scattered); and (iii) high energy secondary electrons.

A primary electron column typically includes the following main components: an electron gun (source of primary electrons); a primary imaging system (e.g., a primary electron projection imaging system) with an aberration correction component; and a primary electron beam scanning unit. In the electron gun, primary electrons are generated by emission from the cathode tip; these primary electrons are then accelerated to the energy necessary to transport them through the electron optical system of the SEM. The primary electron projection imaging system transports the primary electrons from the source to the wafer. The primary electron projection imaging system performs focusing of the electron beam on the sample surface and adjusts the final kinetic energy before the electrons are incident on the sample surface. The electron beam scanning deflection unit performs beam scanning of primary electrons across the detection area.

The secondary column collects the secondary electrons generated at the sample surface and transports them to a detector. The secondary column can be simple, having only a detector positioned close to the sample surface. Or it can be more complex, having components such as a backscanning deflection unit, a projection imaging system with aberration correction components for the secondary electrons, and a detector.

A secondary imaging system (e.g., a secondary electron projection imaging system) can project secondary electrons originating from a detection spot on a sample surface onto a detector. The secondary imaging system can be designed to maintain the position and focus of the image on the detector independently of the imaging parameters of the system (e.g., landing energy). The secondary backscanning deflection unit can be configured to remove image displacements on the detector due to the moving position of the detection spot (the origin position of the secondary electrons).

In the semiconductor industry, the success of detection and metrology tools based on the principle of scanning electron microscopy can be judged primarily by the short wavelength and high scattering cross section of the accelerated electrons with a kinetic energy of 100 eV to 100 keV. The short wavelength can help to achieve a high resolution of the acquired images, while the high scattering cross section leads to a high intensity of the detected signal and thus a good signal-to-noise ratio (SNR).

At the same time, some tools can suffer from low measurement speeds. For example, to obtain a surface image, an electron beam may be scanned across all points of the inspection area. Scanning a single beam focused to a spot size of a few nanometers across a wafer (e.g., a 30 cm diameter wafer with a surface area of several square centimeters) or even just a relatively small pre-selected area can be extremely time consuming. As a result, single beam systems can face difficulties in providing the necessary throughput required by semiconductor fabs for conventional in-line wafer inspection.

High throughput of inspection tools using the principles of scanning electron microscopy can be achieved by utilizing multiple electron beams in parallel to simultaneously image multiple sections of the inspection area. Secondary electron microscopes that use multiple electron beams in parallel for imaging semiconductor structures may be referred to as multi-beam SEMs and are of great interest to the semiconductor industry. Examples of such multi-beam SEMs may be found in U.S. Patent Nos. 9,691,588 and 10,141,160, which are incorporated herein by reference in their entirety.

Referring now to FIG. 2 , a multi-beam device consistent with an embodiment of the present invention is illustrated. The electron optics design of a multi-beam SEM may include two main sections: a primary electron column; and a secondary electron column. The electron beam tool 100 may be an example of a charged particle beam device, which may be part of an EBI system 10, as discussed above with respect to FIG. 1 .

The primary column may include electron optical components associated with primary electron generation, components for forming a plurality of primary electron beamlets, components for transporting the primary electron beamlets to the sample surface, and a scanning deflection unit that can be used to scan the primary beamlets across a detection area. The electron source of a multi-beam SEM can be different than a conventional single-beam electron source. For example, a multi-beam SEM can be configured to generate a plurality of electron beamlets (e.g., an array of electron beamlets). This can be achieved by supplementing a conventional electron gun with a conversion unit to generate a plurality of virtual electron sources. An example of a multi-beam system that generates a plurality of beamlets is discussed in U.S. Patent No. 10,141,160.

As shown in FIG. 2 , the electron beam tool 100 may include an electron source 101, a main aperture plate 171, a focusing lens 110, and a conversion unit 120. The electron source 101, the main aperture plate 171, the focusing lens 110, and the conversion unit 120 may be included in a primary electron column. The electron source 101 may be configured to generate a primary electron beam 102 along a main optical axis 100_1. The conversion unit 120 may include a microelectromechanical system (MEMS) structure. For example, the conversion unit 120 may include a beam limiting aperture, a microdeflector array, and a microlens array. The electron source 101 may generate a primary electron beam 102 that is transmitted through the focusing lens 110. The peripheral electrons of the primary electron beam 102 can be cut off using the main aperture plate 171.

As shown in FIG2 , a plurality of beamlets 102_1 , 102_2 and 102_3 can be generated and can be directed toward a sample 8. Produced by an electron gun and a conversion unit, an array of nearly parallel electron beamlets can be delivered to the surface 7 of the sample 8 by a single-column primary projection imaging system. Each beamlet can be focused on a corresponding sub-area of the detection area. A plurality of primary electron beamlets impacting the sample surface can generate a plurality of detection light spots emitting secondary electrons. A common scanning deflection unit performs a scan of the plurality of beamlets across a sub-area of the area under investigation. For example, the electron beam tool 100 may include a beam splitter and a scanning system. The beam splitter may include a Wayne filter. As shown in FIG2 , a beam splitter 160 may be provided. A scanning unit (e.g., an electrostatic deflector) may be configured to deflect the beamlets to achieve scanning. As shown in FIG2 , a scanning deflection unit 132 may be provided. The beam splitter 160 and the scanning deflection unit 132 may be aligned with the main optical axis 100_1. The scanning deflection unit 132 may deflect the beamlets 102_1, 102_2, and 102_3 together so that an array of beam spots scans across the surface 7 of the sample 8. The beam spots formed by the beamlets 102_1, 102_2, and 102_3 may be configured to have a spacing P that may be maintained when performing a scan.

A single secondary column may be provided and may be configured to collect particles of a plurality of secondary electron beamlets originating from a plurality of detection spots within a sub-region of the detection area and transport them to a detector plane. The secondary column may include an objective lens, a beam splitter, a secondary electron projection imaging system with a backscanning deflection unit, and a detector. There may be an overlap between the primary column and the secondary column. Some components of the electron beam tool 100 may be included in both the primary column and the secondary column. For example, the objective lens 131, the beam splitter 160 and the scanning deflection unit 132 may be included in both the primary column and the secondary column.

Although some components such as the beam splitter 160 may not be involved in the scanning process, electrons may still travel through them. For example, in some embodiments, a Wayne filter acts as a beam splitter. The Wayne filter can transmit primary electrons without affecting the trajectory of the primary electrons traveling toward the wafer. This can be achieved by balancing the electrostatic force against the magnetic force (e.g., the Lorenz force) acting on the traveling electrons. The two fields are oriented perpendicular to the main optical axis. On the path back from the wafer to the detector, the Lorenz force will change sign because it can depend on the direction of the electron's motion, while the electrostatic force will act in the same direction. Both forces will act on the electrons in the same direction (orthogonal to the primary axis). As a result, they will deflect the secondary electrons towards the detector (e.g., along the secondary axis). The Wayne filter can be held under static excitation during scanning.

Scanning can be performed by an electrostatic deflector (e.g., a scanning unit). In some embodiments, the electrostatic deflector may include two metal plates, wherein a voltage is applied between the plates to generate an electric field that deflects electrons in a direction perpendicular to the main optical axis. Multiple pairs of metal plates may be provided.

As shown in FIG2 , a secondary imaging system 6 and a detector 7 may be provided. The secondary imaging system 6 may be an example of a projection imaging system. The secondary imaging system 6 and the detector 7 may be aligned with the secondary optical axis 150_1. The secondary column may include an objective lens 131, a beam splitter 160 and a scanning deflection unit 132, the secondary imaging system 6 and the detector 7.

FIG3A shows an example of an enlargement of a portion of a secondary column consistent with an embodiment of the present invention. The secondary imaging system 6 may include a backscanning deflection unit 157, a zoom lens 151, and a projection lens 152. The backscanning deflection unit 157 may include a deflector configured to deflect secondary electrons. The zoom lens 151 may include a plurality of electrostatic lenses. The projection lens 152 may include an electrostatic lens or a magnetic lens.

2 , the objective lens 131 can collect secondary electrons emitted from the detection light spot and can form a small beam of secondary electrons (“secondary electron beamlet”). The beam splitter 160 can deflect the secondary electron beamlet toward the secondary imaging system 6. The secondary imaging system 6 can be used to project the secondary electrons onto the detector 7 and make the focus, size and rotation of the image on the detector 7 almost constant and independent of the SEM imaging conditions.

As shown in Figure 3A, the secondary imaging system 6 may include components that affect the beam traveling through it. The components in the secondary imaging system 6 may affect the beamlets together. For example, the backscan deflection unit 157 may shift the incoming array of secondary electron beamlets relative to the secondary optical axis 150_1, which is common to all beamlets entering the secondary imaging system 6. The backscan deflection unit 157 may be configured to minimize the image displacement on the detector 7. The image displacement on the detector 7 may result from the movement of the detection light spot across the detection area on the surface 7 of the sample 8. The backscan deflection unit 157 may affect the beam synchronously with the electron deflection scanning system of the primary electron beamlets. For example, the backscan deflection unit 157 can affect the beamlets synchronously with the scan deflection unit 132. The detector 7 can detect electrons coming as a plurality of beamlets. The detector 7 can be assembled from individual sensors, or can be implemented as one detector with a plurality of sensitive elements (e.g., detector segments or groups of sensitive elements, such as groups of small pixels). Each secondary beamlet can be configured to be projected onto a corresponding detector or detector segment.

FIG3B shows another example of a configuration of a secondary imaging system consistent with an embodiment of the present invention. As shown in FIG3B , a plurality of deflectors may be provided for backscanning the secondary electron deflection unit. For example, at least two deflectors 157_1 and 157_2 may be provided. Deflector 157_1 may be configured to deflect the beamlet toward the secondary optical axis 150_1, and deflector 157_2 may be configured to reversely deflect the beamlet so that it has a trajectory parallel to the secondary optical axis 150_1.

In some embodiments, the secondary electron projection imaging system may include a combination of zoom lenses (including at least two lenses), anti-rotation lenses, and aberration compensating electronic optical elements (e.g., astigmatism correctors). Each of the lenses may be an electrostatic lens, a magnetic lens, or an electrostatic-magnetic composite lens. In addition, each lens may be implemented as a single lens or as a whole of lenses (e.g., some lenses that perform coarse focusing and some lenses that perform fine focusing). The above situation may be applicable to aberration compensating electronic optical elements.

In a comparative embodiment, all components of the reprojection imaging system may be selected to provide the best average performance for a plurality of beamlets, and the excitations of the components are fixed to specific values during a scan.

The SEPI system can be configured to keep the magnification, focus and rotation of the image on the detector constant. These parameters (magnification, focus, image rotation) should be independent of the imaging parameters of the multi-beam system (e.g., landing energy, parameters controlling the SE emission pattern, etc.).

A plurality of secondary electron beamlets originating from a plurality of detection spots can be transported and focused to a detector. The detector can perform detection of electrons by utilizing the established one-to-one correspondence between a specific sub-area of the detection region and one of the detection units.

The performance of the secondary column in a multi-beam charged particle device can be determined by two main parameters: the collection efficiency and the crosstalk. The collection efficiency of a single detector unit can be defined as the fraction of secondary electrons emitted from the corresponding detection light spot in one of the sub-areas of the detection area and detected by the corresponding detection element. The collection efficiency can be initially defined for each detection element. In some embodiments, in order to characterize the entire detector, an average value of the collection efficiency of all detection elements can be derived. In some embodiments, the collection efficiency can be a numerical complement to characterize the spread of values between elements (such as minimum and maximum values across all elements of the detector).

Crosstalk can be defined as the fraction of secondary electrons detected by an individual detector cell that do not originate from the corresponding subregion of the scanned area but rather from a neighboring subregion. Similar to the collection efficiency, the average, minimum, and maximum values across all elements of the detector can be calculated for characterization of the entire detector.

High collection efficiency and low crosstalk may be important design parameters for achieving high resolution and high throughput of multi-beam charged particle devices to be used as defect inspection tools at semiconductor fabrication plants.

Reference will now be made to Figures 4A, 4B, 5A and 5B, which illustrate scanning of a primary electron beam consistent with an embodiment of the present invention. Figures 4A and 4B show a side view of a portion of a primary electron column. Beamlets 102_1, 102_2 and 102_3 may be formed in the primary electron column and may travel parallel to the main optical axis 100_1. Beamlets 102_1, 102_2 and 102_3 may impact a sample 8 and form beam spots 102_1S, 102_2S and 102_3S. The scanning deflection unit 132 may be configured to deflect the beam traveling therethrough. As shown in Fig. 4B, the scanning deflection unit 132 can deflect the beamlets 102_1, 102_2 and 102_3 so that the beam spots 102_1S, 102_2S and 102_3S move across the sample 8. Fig. 5A and Fig. 5B show corresponding top views of the sample 8.

In the scanning mode of operation of the multi-beam SEM, the primary deflection scanning unit continuously moves the array of primary beamlets across the detection area. For example, the scanning deflection unit 132 can deflect the beamlets 102_1, 102_2 and 102_3 so that the beam spots 102_1S, 102_2S and 102_3S move from position A to position B, as shown in Figure 5B. The beamlets can be deflected together so that the beam spots on the sample surface move as a unit. Secondary electrons can be generated at each position where a corresponding detection spot is formed on the surface of the sample. During the scan, the origin position of the secondary electron beamlets can be continuously changed during operation, and the path of the secondary electron beamlets through the electron optical system also changes. However, in comparative multibeam SEMs, the excitation of the electronic optics of the secondary projection imaging system (e.g., lenses, astigmatism correctors, and other aberration compensation elements) is optimized only for the undeflected position of the beamlets and maintained there during scanning.

FIG. 6 illustrates a scanning area and a scanning path consistent with an embodiment of the present invention. As shown in FIG. 6 , a scanning area 400 may be provided on the surface of a sample. The scanning area 400 may correspond to the FOV of a charged particle beam device. The scanning area may be within the FOV of the charged particle beam device. During the scanning operation, the primary charged particle beam may be scanned in a grating pattern to cover the scanning area 400. In some embodiments, a plurality of beamlets may be used, and the scanning path of each beamlet may correspond to the scanning path shown in FIG. 6 , or the scanning path of an array of beamlets may correspond to the scanning path shown in FIG. 6 . The scanning path may start at point C1 and continue in a sawtooth pattern or some other pattern. Points C1, C2, C3 and C4 may be the corners of the scanning area 400. Points C1, C2, C3 and C4 may be the fully deflected positions of the beam. Point C5 may be at the center of the scanning area 400. Point C5 may be the undeflected position of the beam. For example, when the primary beam is at point C5, the secondary imaging system may be optimized for the condition of an undeflected beam. When the primary beam is at points C1, C2, C3 or C4, secondary charged particles may be generated by the beam spots at these points and travel from them to the secondary imaging system.

As shown in FIG. 5B and FIGS. 3A to 3G , the secondary beamlets originating from position B may be offset relative to the secondary optical axis 150_1 when they enter the secondary imaging system 6. The backscanning deflection unit 157 of the secondary imaging system 6 may be configured to compensate for such offsets and may redirect the beamlets to align with the optical axis 150_1 so that the trajectory of the beamlets may be similar to the beamlets originating from position A (e.g., undeflected beamlets). The backscanning deflection unit 157 may eliminate the shift of the secondary beamlet array due to the scanning. It should be understood that the backscanning deflection unit 157 may not need to deflect undeflected beamlets (e.g., beamlets originating from position A).

However, even when the offset is corrected (e.g., by shifting the deflected beamlet), some properties of the beamlet may not be the same as the undeflected beamlet. For example, a secondary beamlet originating from a deflected position within the scan area 400 (e.g., at point C1, C2, C3, or C4) may have an optical path that is different from the optical path of an undeflected beamlet (e.g., a secondary beamlet originating from point C5). The corresponding trajectories of the secondary electrons included in the secondary beamlet may have different lengths and pass through different points within the electron optical components (e.g., lens Wayne filter, etc.) of the secondary projection imaging system. The secondary beamlets may be affected in different ways by the electron optical components and experience different aberrations. If the excitation of the components of the secondary imaging system remains the same during a scan, these effects are not compensated by the lenses and astigmatism correctors, so the image produced by the secondary imaging system can become defocused (e.g., the beam spot can be widened) and the image can be affected by directional smearing effects (e.g., astigmatism). The degraded image quality at the detector can result in reduced collection efficiency and degraded crosstalk of the detector unit for a full field of view (FOV) lower than theoretically possible.

As an example, in a comparative embodiment, the secondary imaging system may not be optimally configured to handle secondary charged particles originating from points C1, C2, C3, or C4, as shown in FIG6, and there may be a deviation in image quality compared to the case where the secondary charged particles originate from point C5. The secondary imaging system may be configured to apply certain excitation conditions to the components therein to achieve focusing of the beamlets onto respective regions of the detector.

In comparative embodiments, deviations from the optimal excitation conditions for the electro-optical elements of the reprojection imaging system can be considered small and remain uncompensated. However, under certain imaging conditions, the degradation of the image quality of the imaged detection spots can become significant. In this case, a reoptimization of the reprojection imaging system at each scanning step can improve the image quality on the detector.

In some embodiments of the present invention, a backscanning mode of operation may be provided for a secondary imaging system of a multi-beam device. In some embodiments, the backscanning mode of operation may be applied to various embodiments of multi-beam device designs and secondary imaging systems, such as those discussed in U.S. Patent Nos. 9,691,588 and 10,141,160. The backscanning mode of operation may enhance collection efficiency and reduce crosstalk in the secondary imaging system for all points of the FOV compared to a standard scanning mode.

In a comparative embodiment, a standard scanning sequence may include: 1. A primary scanning deflection unit deflects a primary beamlet that moves a detection spot across a detection region. 2. A secondary backscanning deflection unit is used to eliminate displacement of the secondary electron beamlet array relative to the optical axis on the detector of the secondary imaging system. 3. The secondary imaging system is kept fixed (e.g., the excitation values of the electron optical components in the secondary imaging system are fixed) in a state corresponding to the optimization performed for the undeflected position of the primary beamlet.

In some embodiments, a modified scan sequence may be used that includes a backscanning mode of operation for a secondary imaging system. The modified scan sequence may be similar to the sequence mentioned above for the standard scan sequence, except in step 3, such excitation may be updated synchronously with the primary scan deflection unit performing the scan, or the backscanning deflection unit performing the backscanning, rather than fixing the excitation of the secondary imaging system. Other components of the secondary imaging system may be adjusted in synchronization with the primary scan deflection unit or the backscanning deflection unit. Adjusting the excitation in the backscanning mode of operation may compensate for changes in the initial imaging conditions used for the secondary imaging system caused by the scan. The backscanning mode of operation may perform a backscanning function on the secondary imaging system as a whole. The backscanning mode of operation can enhance the compensation of focus and aberrations in the secondary imaging system at any point in the FOV and can minimize the degradation of the detection spot image on the detector.

In some embodiments, all electron optical components of the secondary projection imaging system can be updated synchronously with the primary scanning deflection unit to act together as a single optical system to eliminate the impact on the image produced by the scanning of the primary electron beamlet. The method of operating the secondary imaging system may include using a backscanning mode of the secondary imaging system. The charged particle beam system may include a backscanning secondary imaging system.

The range of varying stimulus values for the electronic optical components of the secondary imaging system can be determined by theoretical modeling for all detection spot positions within the FOV or scanning area on the sample. In some embodiments, the range of varying stimulus values can be determined experimentally. In some embodiments, interpolation can be used. For example, the number of calculations can be reduced by finding stimulus values for a subset of positions (e.g., center and corners) and using interpolation to determine intermediate values.

Now refer to Figure 3C, which illustrates another configuration that shows an enlargement of a portion of a secondary column consistent with an embodiment of the present invention. Similar to the example of Figure 3A or Figure 3B, as shown in Figure 3C, the secondary imaging system 6 may include a backscanning deflection unit 157, a zoom lens 151, and a projection lens 152. However, the secondary imaging system 6 may also include an aberration compensator 9. The aberration compensator 9 can be configured to compensate for the astigmatism aberration of the multiple secondary electron beams entering the secondary imaging system 6 due to the beam splitter 160. Some components can be configured to compensate for the aberrations due to the objective lens 131. In addition, the projection lens 152 can be configured as a derotating lens. The projection lens 152 can be configured to compensate for the rotation effect induced by other components of the secondary imaging system 6. The components of the secondary imaging system 6 can be configured to maintain a correspondence between the images of the plurality of detection light spots formed by the plurality of primary beamlets and the plurality of detection elements in the detector 7. The detection elements can include individual or groups of sensing elements, or can include different sub-areas of the detector 7. Through the correspondence, each detection element can generate an image signal corresponding to the scanning area.

In some embodiments, the secondary imaging system 6 may further include an alignment deflector to compensate for deviations in the corresponding relationship due to manufacturing or assembly errors of components of the secondary column (including, for example, the detector 7).

In some embodiments, the configuration of components in the secondary imaging system 6 may be changed. For example, the back scan deflection unit 157 may be between the beam splitter 160 (see FIG. 2 ) and the zoom lens 151. The back scan deflection unit 157 may be a leading component in the secondary imaging system 6. In some embodiments, the back scan deflection unit 157 may be between the zoom lens 151 and the detector 7. In some embodiments, the back scan deflection unit 157 may be between individual lenses of the zoom lens 151.

Additional examples of different configurations of components in the secondary imaging system 6 are shown in Figures 3D to 3G. For example, Figure 3D shows a configuration in which all back-scanning deflectors are positioned before the zoom lens 151 (e.g., upstream of the zoom lens 151). In some embodiments, the configuration of Figure 3D can be configured to achieve an optimal scenario for the execution of the secondary imaging system 6. Figure 3E shows a configuration that may be applicable due to spatial limitations before the zoom lens 151. As shown in Figure 3E, at least one back-scanning deflector 157_1 may be positioned before the zoom lens 151, and at least one back-scanning deflector 157_2 may be positioned after the zoom lens 151. Figure 3F shows a configuration that may be applicable due to additional spatial limitations. As shown in Fig. 3F, at least one anti-scanning deflector 157_2 may overlap with the first zoom lens of the zoom lens 151. Fig. 3G shows a configuration that may be applicable due to other space constraints. As shown in Fig. 3F, all anti-scanning deflectors may be positioned behind the first zoom lens of the zoom lens 151.

In operation, secondary particles generated by the beam spot on the sample may be incident into the secondary imaging system 6 from position A, as shown in FIG. 5B and FIGS. 3A to 3G. Position A may correspond to an undeflected position of the array of primary beamlets on the sample, and the secondary beamlets incident into the secondary imaging system 6 may be undeflected. Position B may correspond to a deflected position of the array of primary beamlets on the sample, and the secondary beamlets incident into the secondary imaging system 6 may be deflected. The backscanning deflection unit 157 may deflect the beamlets from position B in order to correct the offset. The beamlets may be reversely shifted so that they are aligned with the secondary optical axis 150_1. The backscanning deflection unit 157 may perform backscanning synchronously with the scanning deflection unit 132 (see FIG. 2 ). Together with the scanning of the primary beamlet, the secondary beamlet can be backscanned. In addition, other components can be operated in a backscanning mode. Other components in the secondary imaging system 6 can be operated with an excitation that is adjusted synchronously with the backscanning deflection unit 157 or the scanning deflection unit 132.

Excitation of the components of the secondary imaging system 6 may include, for example, adjusting the electrical signal applied to the lens. The excitation may include a voltage or current of a driver. For example, the voltage applied to the electrostatic lens of the zoom lens 151 may be adjusted. Adjustment of the excitation of the electrostatic lens may include changing the voltage applied to the electrodes of the electrostatic lens. The compound lens may include electrodes to which the voltage can be adjusted. Adjustment of the excitation may be based on the deflection setting of the scanning deflector or the anti-scanning deflector. In some embodiments, the deflection setting of the scanning deflector may correspond to the deflection setting of the anti-scanning deflector. The components of the secondary imaging system 6 can be adjusted based on the deflection setting of the backscanning deflection unit 157. For example, when the backscanning deflection unit 157 deflects the secondary beamlet from position B to align with the secondary optical axis 150_1 (e.g., similar to the secondary beamlet from position A), the excitation of the zoom lens 151 can be adjusted. The degree of adjustment of the excitation can be based on the relationship between the amount of deflection and the impact on image quality (e.g., the expected amount of aberration caused by the deflection). In some embodiments, the degree of adjustment of the excitation can be proportional to the amount of deflection.

In some embodiments, the adjustment of the excitations of the components of the secondary imaging system 6 can be performed one by one. For example, the excitations applied to the zoom lens 151, the projection lens 152, and the aberration compensator 9 can be adjusted one by one. In some embodiments, the excitations applied to all components of the secondary imaging system 6 can be adjusted together. For example, the excitations applied to the zoom lens 151, the projection lens 152, and the aberration compensator 9 can be adjusted together with the operation of the backscanning deflection unit 157.

In some embodiments, for example, when the backscanning deflection unit 157 is upstream of other components in the secondary imaging system 6 to be adjusted, the excitation applied to the other components can be adjusted together. In some embodiments, for example, when other components are upstream of the backscanning deflection unit 157, these components can be adjusted in a different manner than components downstream of the backscanning deflection unit 157. For example, the lens upstream of the backscanning deflection unit 157 can be adjusted to compensate for the aberration changes attributable to the deflection of the beamlet. The beamlet entering such a lens may not have its deviation corrected by the backscanning deflection unit 157, and the beamlet may experience additional distortion. Such distortions can be compensated by adjusting the excitation of the lens synchronously with scanning the primary beamlet. For example, the lens provided as a leading component of the secondary imaging system 6 can be configured to compensate for aberrations due to the deflected beamlet passing through the objective lens 131 and the beam splitter 160 before reaching the secondary imaging system 6.

Although illustrated as parallel in FIGS. 3A to 3G , in some embodiments, a deflected secondary beamlet (e.g., from position B) may be incident into the secondary imaging system 6 at an angle different from the angle of an undeflected beamlet (e.g., from position A). For example, a beamlet from position B may enter the secondary imaging system tilted relative to the secondary optical axis 150_1. The backscanning deflection unit 157 may shift the beamlet so that its offset is eliminated, however, the trajectory of the beamlet may still be tilted relative to the secondary optical axis 150_1. In some embodiments, components of the secondary imaging system 6 may be configured to correct the tilt of the incident deflected beamlet. The backscanning deflection unit 157 may be configured to eliminate not only the offset but also the tilt.

Now refer to Figures 7A to 7C, which illustrate the projection of the beam spot image on the detector unit of the detector in accordance with an embodiment of the present invention. Figures 7A to 7C can schematically show the distribution of electron spots on the detector 7 generated by the secondary imaging system 6 via contour lines. The contour lines of the electron spot distribution can be used to characterize the size and shape of the spot. Figure 7A illustrates the spot on the detector that can correspond to the undeflected position of the detection spot within the FOV of the charged particle beam device. As shown in Figure 7A, the spot on the detector typically has a circular shape and is well focused. This can be attributed to the fact that the excitation of the components of the secondary imaging system is fully optimized. FIG. 7B illustrates the light spots on the detector which may correspond to the case where the primary electron beamlet is deflected to one of the FOV corners. At the same time, the excitation of the components of the secondary imaging system may remain fixed (e.g., using a standard scanning mode). In the case of FIG. 7B , the light spots are larger and have a strongly distorted shape compared to FIG. 7A . The distortion of the light spots in FIG. 7B may be attributed to the fact that the focusing and compensation of the aberrations remain the same as in the case of FIG. 7A . As shown in FIG. 7B , some of the light spots are larger than the individual cells of the detector. This may result in low collection efficiency and considerable crosstalk.

In some embodiments, a backscan mode of the secondary imaging system may be used. For example, the secondary imaging system 6 (see Figures 3A to 3G) may be configured to completely reoptimize the excitation of the components of the secondary imaging system 6 based on the deflection of the beamlet entering the secondary imaging system 6. As shown in Figure 7C, the light spot on the detector 7 corresponding to the deflected beamlet may be similar to the light spot corresponding to the undeflected beamlet (see Figure 7A). The backscan mode allows the secondary imaging system 6 to achieve better collection efficiency and smaller crosstalk values compared to standard operation. This can achieve higher resolution and higher speed operation. For example, high-speed and high-resolution measurements can be performed in an electron beam inspection process. For defect detection systems, higher throughput, sensitivity, and accuracy can be achieved.

Backscanning can be performed without using a feedback loop. For example, in a comparative embodiment, a monitoring system can be provided that images a beam spot projection pattern on a detector and then performs correction based on the imaged projection pattern. In some embodiments of the present invention, a feedback loop that uses an image of the beam spot projected on the detector can be omitted. Dynamic adjustment (e.g., dynamic focusing) of components of the secondary imaging system can be achieved without the need for a monitoring system and a feedback loop. By eliminating the feedback loop, substantially all detection signals generated by the beam spot on the detector can be used for signal generation, and the SNR can be enhanced. Furthermore, the backscanning mode of operation of the secondary imaging system can be directly applied to the charged particle beam device without the need for additional hardware, such as a monitoring system.

In some embodiments, a secondary imaging system such as that discussed in U.S. Patent Nos. 9,691,588 and 10,141,160 may be used. It should be understood that in some embodiments, additional variations may be used, such as placing the backscanning deflection unit 157 further downstream of the zoom lens 151, and different configurations of lenses (e.g., integral lenses).

Additionally, in some embodiments, the shape of the beam spot on the detector can be adjusted. Methods for optimizing the electron beam spot on the detector and optimizing the shape and size of the detector cells in a charged particle beam device (e.g., a multi-beam SEM) can be provided. In a comparative embodiment, a circular shape of the electron beam spot can be formed on the detector as a goal. However, other shapes can be used to enhance the performance of the detector. For example, an elongated shape (such as an elliptical shape of the spot on the detector) can help reduce crosstalk and increase collection efficiency. The orientation of the ellipse can be aligned along the diagonal connecting the nearest neighbors of the detector cells. Corresponding methods for optimizing the shape and size of the detector cells of a pixelated detector of a multi-beam device can also be provided.

Some embodiments may relate to the operation of a multi-beam charged particle device that utilizes multiple charged particle beams. One of the implementations of such a device is a high-resolution imaging system using the principles of a multi-beam scanning electron microscope (SEM). In detail, a multi-beam SEM can be implemented as a wafer inspection tool and applied to imaging circuit patterns fabricated on semiconductor wafers. Single-beam SEMs are known to be widely used in the semiconductor industry as wafer inspection tools. The main advantages of SEMs are their high resolution and high sensitivity to surface charge distribution, which makes the SEMs sensitive to a variety of manufacturing defects. However, for surface imaging, these tools utilize a single electron beam that scans across the inspection area and are therefore greatly affected by low image acquisition speeds. In a multi-beam SEM, to increase the measurement speed, the inspection area is split into multiple sub-areas that are all scanned simultaneously by multiple focused electron beams. Such systems can achieve high throughput and high resolution and can be a useful tool for wafer and mask inspection in the semiconductor industry.

In a single beam electron system (e.g., a SEM), an electron beam with a circular cross-section may be used. The electron beam may be centered on the optical axis of the system and have rotational symmetry about the optical axis. Unless the shape of the beam is distorted by aberrations or affected by optical elements (e.g., by astigmatism correctors or deflectors), the minimum footprint on the image plane (and therefore the best resolution) is usually obtained for an electron spot of circular shape. Naturally, the circular shape of the electron spot on the image plane (e.g., sample surface, detector surface) may be used as a target shape for optimizing system performance.

In some embodiments, for example, for a multi-beam SEM, the circular shape of the detector electron spot may not be the optimal shape to obtain the best achievable performance of the system. Shapes other than a circular shape may be targeted.

A method of optimizing an electron beam spot on a detector of a charged particle beam device (e.g., a multi-beam SEM) may be provided. Parameters of the electron beam spot may be adjusted. The parameters may include, for example, the size, shape, or configuration of the electron beam spot. An elongated shape of the spot on the detector may be used to minimize crosstalk and maximize collection efficiency values. The elongated shape may be oriented along a diagonal connecting the nearest neighbors of the detector cells. The elongated shape may include an ellipse.

Reference is now made to Figures 8A and 8B, which illustrate circularly shaped beam spots consistent with embodiments of the present invention. In a comparative embodiment, a standard optimization may be represented by Figure 8A. The standard optimization may include: (i) the spot is optimized to have a circular shape; (ii) the diameter ( _d1 ) of the spot is minimized. Figure 8A shows such an optimization for a detector in which the individual detector cells are arranged side by side (without spacing) in a grid. Figure 8B shows the same standard optimization for a detector in which the cells are arranged in a grid with gaps between the cells. In a detector assembled from individual components, the "side by side" configuration or the configuration with gaps between the cells can be determined in the system design. In the case of a 2D pixelated detector morphology, the size and shape of individual detector cells and the spacing between individual detector cells can be adjusted during measurement. The selection of these parameters can become part of a method for optimizing the detection performance of the system.

Reference is now made to Figures 9A and 9B, which illustrate beam spots of non-circular shapes consistent with embodiments of the present invention. In some embodiments, the process for optimization may include: (i) the spot is optimized to have an elongated (e.g., elliptical) shape, where the elongation is oriented along the line connecting adjacent diagonal elements of the detector; (ii) the size of the minor axis (the minor axis perpendicular to the line) is minimized, while the major axis (long axis) can be slightly longer. In the case of a regular arrangement of the detector elements in a square N×N matrix array, the major axis can be approximately 1.4 times larger than the minor axis. Figure 9A shows square detector cells arranged side by side in a grid. Figure 9B shows square detector cells arranged in a standard grid with gaps between cells.

Using non-circular shapes, a larger spot size along the diagonal direction can be adjusted by considering the distance between adjacent elements in the N×N square matrix array. In some embodiments, the distance between adjacent detector elements positioned on the diagonal is larger than the distance between adjacent detector elements positioned horizontally or vertically by a factor of √2 (square root of 2). The spot size along the diagonal direction can be made larger without increasing the overlap between corresponding spots. The spot size can be increased without increasing crosstalk and without reducing collection efficiency. For example, the criteria for optimizing the shape of the beam spot can be relaxed in this regard without having a deleterious effect on crosstalk and collection efficiency.

Reference is now made to Figures 10A and 10B, which illustrate beam spots of non-circular shapes consistent with embodiments of the present invention, and detector cells in an offset pattern. In some embodiments, the spots and corresponding detector cells may be arranged in an N×N matrix having a shape that is close to a rectangle or a parallelogram. The detector cells may be arranged in a lattice form, wherein the elements of the lattice have a parallelogram shape. The procedure for optimization may include: (i) the spots are optimized to have an elliptical shape, wherein the ellipse is oriented along a line connecting adjacent diagonal elements of the detector; (ii) the size of the minor axis (the short axis perpendicular to the line) is minimized, while the major axis (the long axis) may be slightly longer. In this case, the scaling factor of the major axis (oriented along the diagonal) relative to the minor axis (oriented perpendicular to the diagonal) can be estimated, for example, as √(a^2+b^2)/c. In some embodiments, the scaling factor can be estimated as (1/2*(a+b))/c. In some embodiments, the scaling factor can be estimated as a/c. The cell shape and size can be adjusted to take advantage of larger cell spacing and distortion of the cell array in different directions, as shown in FIG. 10B. Furthermore, in some embodiments, the light spot can be optimized to have an elongated shape that is elongated along the lines connecting adjacent lattice elements of the detector array. For example, as shown in Figure 10B, a plurality of detector cells may be arranged in an array such that the lattice elements of the array have a parallelogram shape and the detector cells have a rectangular shape. The lines connecting adjacent lattice elements may not necessarily align with the diagonals traced from the corners of individual detector cells.

Reference is now made to Figures 11A and 11B, which illustrate beam spots that may have asymmetric distributions consistent with embodiments of the present invention. For spots that have an asymmetric distribution relative to the center of the geometric spot (e.g., due to coma), the detector cells may be shifted to cover the largest portion of the spot and leave only a smaller end of the spot distribution outside the cell (see Figure 11A). Gaps between cells may be provided and left open to minimize crosstalk between elements. The location of the cell boundaries may be determined using the projected intensity distribution of the spot on the corresponding axis connecting adjacent detector cells. In some embodiments, the boundary location may be optimized based on the ratio between collection efficiency and crosstalk. In some embodiments, this ratio may be maximized. A specific value of crosstalk between neighboring elements (e.g., 1% to 5%) can be used as a starting condition for setting the cell boundaries, and in a second step, the collection efficiency and crosstalk ratio can be maximized by shifting the boundaries back and forth in the direction connecting the neighboring elements. In some cases, the cell shape can be defined not only by the four nearest neighbors but also by additional spaces along the diagonal that can be used to minimize crosstalk (see Figure 11B). These new boundaries (shown by dotted lines in Figure 11B) can define additional spacing along the diagonal direction and will cut the corners of the cell.

Referring now to FIG. 12 , an approach to determining detector cell shape and size consistent with an embodiment of the present invention is illustrated. A general approach for defining the shape of a cell on a pixelated detector may be represented by FIG. 12 . The procedure for determining cell shape and size may include: Drawing rays from the center of the spot in all possible directions; For each ray, calculating the cross-section of the 2D spot distribution, and setting the cell boundaries according to the target crosstalk value; Maximizing the collection efficiency and crosstalk ratio by shifting the cell boundaries back and forth along the ray direction; After finding the cell boundaries in all directions for all detector cells, the most general form of the detector cell may be determined for the pixelated detector.

Components of the secondary imaging system may be used to control parameters of the beam spot on the detector. For example, the secondary imaging system 6 may include components that affect the beam traveling through the secondary imaging system and have an effect on the size and shape of the beam spot formed at the surface of the detector 7. The projection lens 152 may perform a derotation function and may be used to direct the array of spots on the detector. In some embodiments, the objective lens 131 may include a magnetic element, and the beam passing through the objective lens 131 may be subjected to a rotation due to the magnetic field generated by the objective lens 131. The rotation may be around the main optical axis 100_1. The projection lens 152 may be configured to cancel the rotation attributable to the objective lens 131. The projection lens 152 may include a magnetic element (e.g., a magnetic lens) and may rotate the beam passing through the projection lens 152. The rotation may be about the secondary optical axis 150_1. The rotation induced by the projection lens 152 may be opposite in direction to the rotation induced by the objective lens 131. In some embodiments, the projection lens 152 may be configured to allow a predetermined amount of rotation.

Other components of the charged particle beam system can also induce rotation at various points in the system.

The secondary imaging system 6 can be configured to control the shape and size of the beam spot projected on the detector 7, or other parameters. The target shape of the beam spot can be elliptical, and the elliptical shape can be tilted (for example, aligned with a diagonal direction relative to the detector unit). The projection lens 152 can be configured to form a beam spot with a tilted elliptical shape on the detector 7 by controlling the magnetic field generated by the projection lens 152. In some embodiments, the configuration of the detector unit on the detector 7 can be offset (see Figures 10B, 11A, 11B and 12). The projection lens 152 can be configured to form the beam spot on the detector 7 in an array pattern that is rotated to match the offset configuration of the detector unit. The aberration compensator 9 can be configured to adjust the shape and orientation of the beam spot on the detector 7.

The zoom lens 151 can perform a focusing function. The zoom lens 151, the projection lens 152 and the aberration compensator 9 can work together to adjust the focus of the beam passing through the secondary imaging system 6 and can form a focused beam spot image on the detector 7. The components of the secondary imaging system 6 can be adjusted to control the focus of the beam passing through the secondary imaging system 6. The focus can be adjusted to form a beam spot with a target shape. For example, the focus can be adjusted to form an elliptical beam spot.

The control of the components of the secondary imaging system 6 can be based on the deflection settings. For example, the deflection settings can belong to the scanning deflection unit 132, or the backscanning deflection unit 157. For example, the control of focusing and rotation using the zoom lens 151, the projection lens 152 and the aberration compensator 9 of the secondary imaging system 6 can be synchronized with the backscanning using the backscanning deflection unit 157.

In some embodiments, a multi-beam charged particle beam device can be used. In some embodiments, a single-beam charged particle beam device can be used. A single-beam system can also use a scanning deflector. In a comparative embodiment, a backscanning deflector is not used, and the secondary particles are collected by a detector that can have a single detector unit. However, the dispersion of particles affecting the detector can be wide, and therefore a large detector may be required. In some embodiments of the present invention, a backscanning deflector can be used, and the dispersion of particles affecting the detector can be made smaller. In addition, a secondary imaging system can be used, and components of the secondary imaging system can be operated in a backscanning mode. The dispersion of particles can be made smaller by reducing the distortion of the beam spot formed on the detector. Some embodiments of the present invention can achieve miniaturization of detectors or other components.

A method of correcting the focus of a beam may include adjusting an excitation of a component of a secondary imaging system of a charged particle beam apparatus. The beam may be a secondary beam or a plurality of secondary beamlets. The beam may be defocused due to backscanning deflection of the beam. The beam may pass through the secondary imaging system. The beam may pass through the secondary imaging system on the way to a detector or some other component (e.g., a transfer lens).

FIG. 13 is a flow chart illustrating an exemplary method for correcting the focus of a beam consistent with an embodiment of the present invention. The method of FIG. 13 may be performed by, for example, a controller 109 of the EBI system 10 shown in FIG. 1 . The controller 109 may be programmed to implement one or more blocks of the flow chart illustrated in FIG. 13 . For example, the controller 109 may direct a module of a charged particle beam system to generate a charged particle beam and perform other functions. The controller 109 may control the actuation of a beam splitter 160, a scanning deflection unit 132, or a secondary imaging system 6.

The method consistent with FIG. 13 may start with a first step S110 of generating a charged particle beam by a charged particle beam source. For example, a primary beam source including an anode and a cathode may generate a charged particle beam, as shown in FIG. 2 . The first step S110 may include a step S111 of forming beamlets. A plurality of beamlets (e.g., beamlets 102_1, 102_2, and 102_3) may be formed from a primary beam (e.g., primary beam 102).

As shown in Figure 13, a second step S120 can be performed. The second step S120 may include projecting a beam onto the sample. The beam may be the beam generated in the first step S110. The second step S120 may be performed using an assembly of a primary column of a charged particle beam device. For example, an objective lens 131 may be used to project the beam onto the sample 8. The second step S120 may include a step S121 of scanning the beam across the surface of the sample. The second step S120 may include using a scanning deflection unit 132 to scan the sample 8. The beam may be deflected when the scan is performed. There may be a deflection setting of the scanning deflection unit 132 corresponding to performing the scan. For example, a first deflection setting may correspond to a beam in position C1 (see FIG. 6 ), and a second deflection setting may correspond to a beam in position C2 (see FIG. 6 ).

In addition, as shown in Figure 13, a third step S130 may be performed. The third step S130 may include focusing a beam so that the beam spot is projected onto the detector. The beam may be a secondary beam generated by projecting the beam of step S120 onto the sample, and the beam spot is formed on the sample by which the secondary beam is generated. The secondary beam may include a plurality of secondary beamlets. The third step S130 may include a step S131 of setting the focus of the beam. Step S131 may include setting the initial focus of the beam based on the undeflected state of the beam. In some embodiments, step S131 may include setting the focus based on the initial deflection setting of the beam. The deflection setting of the beam may correspond to the setting used in step S121. The third step S130 may also include a step S132 of performing back scanning. Step S132 may be performed by a back scanning deflection unit of the secondary imaging system (e.g., the back scanning deflection unit 157 of Figures 3A to 3G). The back scanning of step S132 may be performed synchronously with the scanning of step S121. The third step S130 may also include a step S133 of adjusting the focus. The focus of the beam set in step S131 may be adjusted. Step S133 may include adjusting the excitation of a component of the secondary imaging system. The component may include a plurality of components. The component may include a lens. Step S133 may include changing the voltage or current applied to the lens. In some embodiments, the assembly may include an electrostatic lens, and step 133 may include adjusting a voltage applied to electrodes of the electrostatic lens.

The step S133 of adjusting the excitation of the assembly can compensate for the decrease in the focus of the beam due to the beam having been deflected. The decrease in the focus of the beam can be due to the beam passing through a backscan deflector (e.g., a backscan deflection unit of a secondary imaging system). The decrease in the focus of the beam can be due to the beam originating from the detection spot position on the sample being different from the undeflected beam (e.g., a beam that has not yet undergone deflection for scanning, or a beam in a basic position for scanning) (see undeflected position A versus deflected position B in FIG. 5B ).

The step S133 of adjusting the excitation of the components can be performed synchronously with the step S132 of performing the backscan. For example, as the beam is displaced in the secondary imaging system by the backscan, the components of the secondary imaging system (such as the lens) can have their excitation adjusted. Similar to the scanning or backscanning operation, the adjustment of the excitation can be performed at a high frequency. The excitation of the components of the secondary imaging system can be continuously updated while performing the scan or backscan.

A step S133 of adjusting the excitation of the assembly may be performed to achieve target parameters of a beam spot formed on the detector. The target parameters may include the size, shape, or configuration (e.g., orientation) of the beam spot. For example, step S133 may include adjusting the excitation of the assembly to form an elliptical beam spot aligned with a diagonal direction of a detector cell of the detector.

In step S133, the component may include a plurality of components, and the stimulus applied to each of the plurality of components may be adjusted.

As also shown in FIG. 13 , a fourth step S140 of imaging processing may be performed. Due to the focusing performed in the third step S130, a beam spot may be formed on the detector, and the detector may generate an imaging signal. The imaging signal may be processed, and an image of the inspected sample surface may be generated. The fourth step S140 may also include performing optimization of parameters of the beam spot formed on the detector, or optimization of parameters of a detector unit of the detector. The method of FIG. 13 may be performed repeatedly, and the parameters may be adjusted when the beam focus is repeatedly adjusted. In some embodiments, it may be known in advance what excitation of the components of the secondary imaging system corresponds to achieving target parameters of the beam spot formed on the detector. In some embodiments, the step S133 of adjusting the excitation of the component can be performed without using a feedback loop. In addition, the step of setting the parameters of the detector (e.g., the size, shape or configuration of the detector unit) can be performed independently of the method of Figure 13 or before the method of Figure 13.

For example, the parameters of the beam spot may include the beam spot size, shape, or configuration. The beam spot size may include the radius of the beam spot. In the case where the beam spot has a non-circular shape, the beam spot size may include other measures of size, such as the major or minor axis of an ellipse. The beam spot size may include the area of the beam spot.

The beam spot shape may be circular or some non-circular shape. In some embodiments, the use of an elongated beam spot (such as an elliptical beam spot) may help to enhance collection efficiency and reduce crosstalk. The use of an elongated beam spot may allow certain imaging conditions to be relaxed because it may become acceptable for some types of distortion to be present (e.g., astigmatism, or coma effects). An elongated beam spot may be allowed to distort from a circular shape while still being contained within the area of the detector unit. Again, optimization of the exact shape of the beam spot may help to maximize collection efficiency and minimize crosstalk.

Furthermore, the configuration of the beam spot may include an orientation, pattern, spacing or rotation of the beam spot. The configuration of the beam spot may be based on parameters of a detector cell of the detector, such as the size, shape and configuration of the detector cell. The elongated beam spot may be oriented so that its elongation direction is aligned with the longest dimension of the detector cell. The detector cell may be square and the elongation direction of the elongated beam spot may be aligned with a diagonal of the square detector cell. The detector cell may be rectangular. Furthermore, a scaling factor may be determined between a major axis and a minor axis of the elliptical or elongated shape of the beam spot. In some embodiments, an array of detector cells may be provided and the direction of elongation of the elongated beam spot may be aligned with a line connecting adjacent lattice elements of the detectors (see FIG. 10B ). In this case, the line connecting adjacent lattice elements of the detectors may not necessarily be aligned with the longest dimension of the individual detector cells. In some embodiments, the beam spot may be asymmetric. In some embodiments, there may be gaps between adjacent detector cells. Optimizing the parameters of the detector may include determining the gaps between adjacent detector cells which may be based on a crosstalk cutoff position.

A method of optimizing parameters of a beam spot may include determining an elongated shape of the beam spot and determining an orientation of the beam spot. The beam spot may be elongated so as to have a long direction and a short direction. The long direction may be oriented based on the shape of a corresponding detector cell of the detector. The long direction may be oriented so as to be aligned with the longest dimension of the detector cell (e.g., a diagonal of a square or rectangular detector cell). In some embodiments, determining the shape or orientation may be based on the configuration of the detector cells in an array. In some embodiments, determining the shape or orientation may be based on an asymmetric nature of the beam spot.

FIG. 14 is a flow chart illustrating an exemplary method for optimizing parameters of a beam spot or detector unit consistent with embodiments of the present invention. The method of FIG. 14 may be performed by a controller 109 of the EBI system 10, such as that shown in FIG. 1 . The controller 109 may be programmed to implement one or more blocks of the flow chart illustrated in FIG. 14 . For example, the controller 109 may direct a module of a charged particle beam system to set or adjust excitations of components of a secondary imaging system, configure detectors, and perform other functions.

The method consistent with Figure 14 may begin with a first step S210 of determining the shape of the beam spot. The first step S210 may include determining the elongated shape of the beam spot. The elongated shape may be an ellipse. In some embodiments, the elongated shape may be asymmetric. The first step S210 may include a step S211 of optimizing the shape of the beam spot. The optimization may, for example, be based on minimizing crosstalk, maximizing collection efficiency, or achieving a predetermined ratio of collection efficiency to crosstalk. The optimization may be based on an expected beam projection pattern and a detector configuration. The optimization may be intended, for example, to change the shape of the beam spot from the shape shown in Figure 7B to the shape shown in Figure 7C, or to some other shape that improves performance based on collection efficiency or crosstalk.

As shown in FIG. 14 , a second step S220 may be performed. The second step S220 may include projecting to determine the beam spot orientation. The beam spot orientation may be determined to be aligned with the long dimension of the detector unit. The beam spot orientation may be determined to be aligned with the diagonal direction of the detector unit. The second step S220 may include a step S221 of optimizing the beam spot orientation. Step S221 may be performed together with step S211.

Furthermore, a third step S230 of determining parameters of the detector may be performed. In some embodiments, the detector configuration may be predetermined. The detector may be fixed and its detector cells may not be adjustable. In some embodiments, for example, a pixelated detector may be used and the detector cells of the detector may be adjustable. The third step S230 may include a step S231 of optimizing the parameters of the detector. The parameters may include the size, shape, configuration, etc. of the detector cells of the detector.

The method of Figure 14 may include a fourth step S240 of providing parameters or parameters determined in a previous step. For example, parameters of the beam spot or parameters of the detector may be provided to the charged particle beam device. The fourth step S240 may include a step S241 of adjusting the excitation of components of the secondary imaging system of the charged particle beam device. For example, step S241 may include step S133 of Figure 13. The fourth step S240 may include a step 242 of configuring the detector of the charged particle beam device. Step S242 may include setting the size, shape or configuration of the detector unit of the detector.

The method of Figure 14 may also include a fifth step S250 of performing imaging. The fifth step S250 may include using a primary beam of a charged particle beam device to irradiate the sample.

In some embodiments, a controller may control a charged particle beam system. The controller may include a computer processor. The controller may direct components of the charged particle beam system to perform various functions, such as controlling a charged particle source to generate a charged particle beam and controlling a deflector of a deflector array to deflect the beam. The controller may also perform functions such as determining a focus value in a secondary imaging system, determining scanning and backscanning, adjusting a focus value in a secondary imaging system, performing image processing, etc. The controller may include a storage medium such as a hard drive, a cloud storage, a random access memory (RAM), other types of computer readable memory, and the like. The controller may communicate with the cloud storage. A non-transitory computer-readable medium may be provided that stores instructions for a processor (e.g., a processor of controller 109) to implement beam focusing or other functions and methods consistent with the present invention. Common forms of non-transitory media include, by way of example: floppy disks, flexible disks, hard disks, solid-state drives, magnetic tapes, or any other magnetic data storage media; CD-ROMs; any other optical data storage media; any physical media with a pattern of holes; RAM, PROM and EPROM, FLASH-EPROM or any other flash memory; NVRAM; cache memory; registers; any other memory chips or cartridges; and networked versions thereof.

The block diagrams in the figures may illustrate the architecture, functionality and operation of the systems, methods and computer hardware or software products according to various exemplary embodiments of the present invention. In this regard, each block in the schematic diagram may represent a certain arithmetic or logical operation process that can be implemented using hardware (such as electronic circuits). Blocks may also represent modules, segments or parts of program codes that include one or more executable instructions for implementing a specified logical function. It should be understood that in some alternative implementations, the functions indicated in the blocks may not appear in the order mentioned in the figure. For example, depending on the functionality involved, two blocks displayed in succession may be executed or implemented substantially at the same time, or the two blocks may sometimes be executed in reverse order. Some blocks may also be omitted. For example, step S132 of backscanning and step S133 of adjusting focus may appear in an order determined by the configuration of the components in the secondary imaging system. For example, when the backscanning deflector is upstream of the component of the secondary imaging system to be adjusted, step S132 may be performed before step S133. When the backscanning deflector is downstream of the component of the secondary imaging system to be adjusted, step S133 may be performed before step S132. When multiple components are to be adjusted and the backscanning deflector is between the components, some steps of adjusting focus may be performed before backscanning, and some steps of adjusting focus may be performed after backscanning is performed. In some embodiments, the step S132 of back scanning and the step S133 of adjusting the focus can be performed simultaneously. In addition, steps such as compensating for astigmatism or other distortions can be added. It should also be understood that each block of the block diagram and the combination of these blocks can be implemented by a dedicated hardware-based system that performs a specified function or action, or by a combination of dedicated hardware and computer instructions.

The following clauses may be used to further describe embodiments: 1. A method for correcting a focus of a secondary beam that is defocused due to backscanning deflection of the secondary beam, the method comprising: Adjusting an excitation of a component of a secondary imaging system through which the secondary beam passes based on a deflection setting of a backscanning deflector, wherein the adjustment compensates for a decrease in the focus of the secondary beam due to the secondary beam passing through the backscanning deflector. 2. The method of clause 1, wherein the component comprises a lens of the secondary imaging system, and the adjustment comprises changing a voltage or current applied to the lens. 3. The method of clause 2, wherein the lens comprises an electrostatic lens. 4. The method of any one of clauses 1 to 3, wherein the assembly comprises an electrostatic lens of a zoom lens or a projection lens. 5. The method of any one of clauses 1 to 4, wherein the assembly comprises an aberration compensator. 6. The method of any one of clauses 1 to 5, wherein the secondary beam comprises a plurality of beamlets. 7. The method of any one of clauses 1 to 6, wherein the excitation is adjusted to form a beam spot on a detector having a target parameter. 8. The method of clause 7, wherein the target parameter comprises a size of the beam spot. 9. The method of clause 7 or clause 8, wherein the target parameter comprises a shape of the beam spot. 10. The method of clause 9, wherein the shape of the beam spot is elongated. 11. The method of clause 10, wherein the shape of the beam spot is elliptical. 12. The method of any of clauses 7 to 11, wherein the target parameter includes an orientation of the beam spot. 13. The method of clause 12, wherein the orientation of the beam spot is aligned with a long dimension of a detector unit of the detector. 14. The method of clause 12, wherein the orientation of the beam spot is aligned with a diagonal direction of a detector unit of the detector. 15. The method of any of clauses 1 to 14, wherein the excitation of the component is adjusted to minimize crosstalk. 16. The method of any of clauses 1 to 14, wherein the excitation of the component is adjusted to maximize collection efficiency. 17. The method of any one of clauses 1 to 14, wherein the excitation of the component is adjusted to achieve a predetermined ratio of collection efficiency to crosstalk. 18. The method of any one of clauses 1 to 17, wherein the excitation of the component is adjusted based on the size, shape or configuration of the detector unit. 19. The method of any one of clauses 1 to 18, wherein the excitation of the component is adjusted synchronously with backscanning performed by the backscanning deflector. 20. The method of any one of clauses 1 to 19, wherein the excitation of the component is adjusted synchronously with scanning performed by a scanning deflector. 21. A computer-readable medium storing an instruction set executable by one or more processors of a system to cause the system to perform a method, the method comprising: adjusting an excitation of a component of a secondary imaging system based on a deflection setting of a backscanning deflector, the secondary imaging system being configured to affect a secondary beam passing through the secondary imaging system, wherein the adjustment compensates for a reduction in the focus of the secondary beam due to the secondary beam passing through the backscanning deflector. 22. The medium of clause 21, wherein the component comprises a lens of the secondary imaging system, and the adjustment comprises changing a voltage or current applied to the lens. 23. The medium of clause 22, wherein the lens comprises an electrostatic lens. 24. The medium of any one of clauses 21 to 23, wherein the assembly comprises an electrostatic lens of a zoom lens or a projection lens. 25. The medium of any one of clauses 21 to 24, wherein the assembly comprises an aberration compensator. 26. The medium of any one of clauses 21 to 25, wherein the secondary beam comprises a plurality of beamlets. 27. The medium of any one of clauses 21 to 26, wherein the excitation is adjusted to form a beam spot on a detector having a target parameter. 28. The medium of clause 27, wherein the target parameter comprises a size of the beam spot. 29. The medium of clause 27 or clause 28, wherein the target parameter comprises a shape of the beam spot. 30. The medium of clause 29, wherein the shape of the beam spot is elongated. 31. The medium of clause 10, wherein the shape of the beam spot is elliptical. 32. The medium of any of clauses 27 to 31, wherein the target parameter includes an orientation of the beam spot. 33. The medium of clause 32, wherein the orientation of the beam spot is aligned with a long dimension of a detector unit of the detector. 34. The medium of clause 32, wherein the orientation of the beam spot is aligned with a diagonal direction of a detector unit of the detector. 35. The medium of any of clauses 21 to 34, wherein the excitation of the component is adjusted to minimize crosstalk. 36. The medium of any of clauses 21 to 34, wherein the excitation of the component is adjusted to maximize collection efficiency. 37. The medium of any of clauses 21 to 34, wherein the excitation of the component is adjusted to achieve a predetermined ratio of collection efficiency to crosstalk. 38. The medium of any of clauses 21 to 37, wherein the excitation of the component is adjusted based on a size, a shape, or a configuration of the detector unit. 39. The medium of any of clauses 21 to 38, wherein the excitation of the component is adjusted synchronously with backscanning performed by the backscanning deflector. 40. The medium of any of clauses 21 to 39, wherein the excitation of the component is adjusted synchronously with scanning performed by a scanning deflector. 41. A method of operating a secondary imaging system of a charged particle beam apparatus, comprising: performing a scan of a primary beam on a sample; performing a backscan of a secondary beam generated by the primary beam incident on the sample; and adjusting an excitation of a component of the secondary imaging system while performing the backscan. 42. The method of clause 41, wherein the excitation of the component is adjusted simultaneously with performing the backscan. 43. The method of clause 41 or clause 42, wherein the excitation is adjusted based on a deflection setting of a backscan deflector performing the backscan. 44. The method of any one of clauses 41 to 43, wherein the component comprises a lens of the secondary imaging system, and the adjustment comprises changing a voltage or current applied to the lens. 45. The method of clause 44, wherein the lens comprises an electrostatic lens. 46. The method of any one of clauses 41 to 45, wherein the component comprises an electrostatic lens of a zoom lens or a projection lens. 47. The method of any one of clauses 41 to 46, wherein the component comprises an aberration compensator. 48. The method of any one of clauses 41 to 47, wherein the secondary beam comprises a plurality of secondary beamlets. 49. A method as in any one of clauses 41 to 48, wherein the excitation is adjusted to form a secondary beam spot on a detector having a target parameter. 50. A method as in any one of clauses 41 to 49, wherein the primary beam comprises a plurality of primary beamlets. 51. A computer-readable medium storing a set of instructions executable by one or more processors of a system to cause the system to perform a method, the method comprising: performing a scan of a primary beam of a charged particle beam device on a sample; performing a back scan of a secondary beam generated by the incident primary beam on the sample; adjusting an excitation of a component of a secondary imaging system of the charged particle beam device while performing the back scan. 52. The medium of clause 51, wherein the excitation of the component is adjusted simultaneously with performing the backscan. 53. The medium of clause 51 or clause 52, wherein the excitation is adjusted based on a deflection setting of a backscan deflector performing the backscan. 54. The medium of any one of clauses 51 to 53, wherein the component includes a lens of the secondary imaging system, and the adjustment includes changing a voltage or current applied to the lens. 55. The medium of any one of clauses 51 to 54, wherein the component includes a lens of the secondary imaging system, and the adjustment includes changing a voltage or current applied to the lens. 56. The medium of any one of clauses 51 to 55, wherein the component includes an electrostatic lens. 57. The medium of any one of clauses 51 to 56, wherein the component comprises an electrostatic lens of a zoom lens or a projection lens. 58. The medium of any one of clauses 51 to 57, wherein the component comprises an aberration compensator. 59. The medium of any one of clauses 51 to 58, wherein the secondary beam comprises a plurality of beamlets. 60. The medium of any one of clauses 51 to 59, wherein the excitation is adjusted to form a beam spot on a detector having a target parameter. 61. A charged particle beam apparatus comprising: a charged particle beam source configured to generate a primary beam; a scanning deflector configured to perform a scan of the primary beam on a sample; and a secondary imaging system comprising: a backscanning deflector configured to perform a backscan of a secondary beam generated by the incident primary beam on the sample; and an assembly configured to control the focusing of the secondary beam to compensate for a reduction in the focal point of the secondary beam due to the secondary beam originating from a deflected position on the sample. 62. The device of clause 61, wherein the primary beam comprises a plurality of primary beamlets, the secondary beam comprises a plurality of secondary beamlets, and the component comprises a lens of the secondary imaging system. 63. A method for optimizing a parameter of a secondary beam spot on a detector of a charged particle beam device, the method comprising: determining the parameter based on a collection efficiency or crosstalk of a detector unit of the detector; providing the parameter to the charged particle beam device; and adjusting the excitation of a component of a secondary imaging system of the charged particle beam device based on the parameter. 64. The method of clause 63, wherein the parameter comprises a size, a shape, or a configuration of the beam spot on the detector, the method further comprising: determining an elongated shape of the beam spot; determining an orientation of the beam spot such that an elongated direction of the beam spot is aligned with a direction of the detector unit; and determining the parameter so as to maximize the collection efficiency, minimize the crosstalk, or achieve a predetermined ratio of the collection efficiency to the crosstalk. 65. A computer-readable medium storing an instruction set executable by one or more processors of a system to cause the system to perform a method comprising: determining a parameter of a secondary beam spot on a detector of a charged particle beam device based on a collection efficiency or crosstalk of a detector unit of the detector; providing the parameter to the charged particle beam device; and adjusting an excitation of a component of a secondary imaging system of the charged particle beam device based on the parameter. 66. A method for optimizing a parameter of a detector of a charged particle beam device, the method comprising: determining a shape of a secondary beam spot on the detector; determining an orientation of the beam spot relative to a detector cell of the detector; determining the parameter based on a collection efficiency or crosstalk of a detector cell of the detector; providing the parameter to the charged particle beam device; and configuring the detector based on the parameter. 67. The method of clause 66, further comprising: optimizing the parameter so as to maximize the collection efficiency, minimize the crosstalk, or achieve a predetermined ratio of the collection efficiency to the crosstalk. 68. The method of clause 66 or clause 67, further comprising: determining a gap between adjacent detector cells of the detector. 69. A computer-readable medium storing a set of instructions executable by one or more processors of a system to cause the system to perform a method comprising: determining a shape of a secondary beam spot on the detector; determining an orientation of the beam spot relative to a detector cell of the detector; determining the parameter based on a collection efficiency or crosstalk of a detector cell of the detector; providing the parameter to the charged particle beam device; and configuring the detector based on the parameter.

It should be understood that embodiments of the present invention are not limited to the exact configurations that have been described above and illustrated in the accompanying drawings, and that various modifications and variations may be made without departing from the scope of the present invention. For example, one or more lenses or other optical components may be added to the specific configurations of the exemplary particle optical systems discussed herein at different locations. Optical components may be provided for, for example, magnification, zooming, and image derotation.

6: Secondary imaging system 7: Detector/surface 8: Sample 9: Aberration compensator 10: Electron beam inspection (EBI) system 11: Main chamber 20: Loading/locking chamber 30: Equipment front end module (EFEM) 30a: First loading port 30b: Second loading port 100: Electron beam tool 100_1: Main optical axis 101: Electron source 102: Primary electron beam 102_1: Beamlet 102_1S: Beam spot 102_2: Beamlet 102_2S: Beam spot 102_3: Beamlet 102_3S: Beam spot 109: Controller 110: Focusing lens 120: Rotation Conversion unit 131: objective lens 132: scanning deflection unit 150_1: secondary optical axis 151: zoom lens 152: projection lens 157: reverse scanning deflection unit 157_1: deflector 157_2: deflector 160: beam splitter 171: main aperture plate 400: scanning area A: undeflected position B: deflected position C1: point C2: point C3: point C4: point C5: point d ₁ : Diameter of light spot S110: First step S111: Step S120: Second step S121: Step S130: Third step S131: Step S132: Step S133: Step S140: Fourth step S210: First step S211: Step S220: Second step S221: Step S230: Third step S231: Step S240: Fourth step S241: Step S242: Step S250: Fifth step

The above and other aspects of the present invention will become more apparent from the description of exemplary embodiments with reference to the accompanying drawings.

FIG. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system consistent with embodiments of the present invention.

2 is a diagram illustrating an example charged particle beam apparatus that may be an electron beam tool that may be part of the example electron beam inspection system of FIG. 1 , consistent with embodiments of the present invention.

3A to 3G are enlarged examples of portions of secondary columns consistent with embodiments of the present invention.

4A and 4B show side views of a portion of a primary electron column in which scanning deflection is applied, consistent with an embodiment of the present invention.

5A and 5B show top views of a sample in which a scanning deflection is applied, consistent with an embodiment of the present invention.

FIG. 6 illustrates a scanning area and a scanning path according to an embodiment of the present invention.

7A, 7B and 7C illustrate the projection of a beam spot image onto a detector unit of a detector in accordance with an embodiment of the present invention.

8A and 8B illustrate circular shaped beam spots consistent with embodiments of the present invention.

9A and 9B illustrate non-circular shaped beam spots consistent with embodiments of the present invention.

10A and 10B illustrate non-circular shaped beam spots and detector cells in an offset pattern consistent with an embodiment of the present invention.

11A and 11B illustrate beam spots that may have an asymmetric distribution in accordance with an embodiment of the present invention.

FIG. 12 illustrates a method for determining the shape and size of a detector unit in accordance with an embodiment of the present invention.

13 is a flow chart illustrating an exemplary method for correcting the focus of a beam in accordance with an embodiment of the present invention.

14 is a flow chart illustrating an exemplary method for optimizing parameters of a beam spot or detector unit consistent with an embodiment of the present invention.

6: Secondary imaging system

7: Detector

9: Aberration compensator

150_1: Secondary optical axis

151: Zoom lens

152: Projection lens

157: Backscan deflection unit

157_1: Deflector

157_2: Deflector

A: Undeflected position

B: Deflection position

Claims (10)

A computer-readable medium storing an instruction set executable by one or more processors of a system to cause the system to perform a method, the method comprising: performing a scan of a primary beam of a charged particle beam device on a sample; performing a backscan of a secondary beam generated by the incident primary beam on the sample; adjusting an excitation of a component of a secondary imaging system of the charged particle beam device while performing the backscan. The medium of claim 1, wherein the incentive for the component is adjusted concurrently with performing the backscan. The medium of claim 1 or 2, wherein the excitation is adjusted based on a deflection setting of a backscanning deflector performing the backscanning. A medium as in claim 1 or 2, wherein the component includes a lens of the secondary imaging system and the adjustment includes changing a voltage or current applied to the lens. A medium as in claim 1 or 2, wherein the component includes an electrostatic lens. A medium as in claim 1 or 2, wherein the component comprises an electrostatic lens of a zoom lens or a projection lens. A medium as in claim 1 or 2, wherein the component includes an aberration compensator. The medium of claim 1 or 2, wherein the secondary beam comprises a plurality of beamlets. A medium as in claim 1 or 2, wherein the excitation is adjusted to form a beam spot on a detector having a target parameter. A charged particle beam apparatus comprising: a charged particle beam source configured to generate a primary beam; a scanning deflector configured to perform a scan of the primary beam on a sample; and a secondary imaging system comprising: a backscanning deflector configured to perform a backscan of a secondary beam generated by the incidence of the primary beam on the sample; and an assembly configured to control the focusing of the secondary beam to compensate for a reduction in the focus of the secondary beam due to the secondary beam originating from a deflected position on the sample.